Optical disks such as compact disks (CDs), video compact disks (VCDs) and digital versatile disks (DVDs) are implemented via recording and reproducing apparatuses. When an optical pickup head of an optical disk operates, a light beam emitted by a light source such as laser light source is focused by an object lens to form a light spot on a plane thereof for recording information. With the fast development of optical disks, it is required to speed-up a disk drive to rotate the disk at a high angular velocity in order to enhance the access speed to the disk data. Unfortunately, referring to FIG. 1, an optical disk 10 is often imparted thereto an eccentric feature in the manufacturing process. In other words, there is generally a shift from the center of an ideal circle aperture 101, which is concentric to the disk, to the center of the real circle aperture 102. Such shift is defined as the eccentric amount C. The generic eccentric amount C of the disk will adversely influence the tracking performance of the optical pickup head especially under a high angular speed condition, and thus results in inferior data pickup performance.
When an optical disk is loaded into a disk drive and then clamped by a spindle motor, the center optical disk might be eccentric from the center of the disk drive, thereby resulting in a further eccentric amount. The eccentric amount is an indication reflecting an imbalance in a radial direction of the disk.
The pitch between two adjacent tracks (track pitch) is a constant, for example 1.6 μm for a standard compact disc and 0.74 μm for a digital versatile disc. Since the eccentric amount of the optical disk is significant compared to the tiny track pitch, the eccentric amount becomes a disturbance factor in the conventional servo control system. Especially when the eccentric amount is at a high level, the tracking control will be unstable and the tracking performance is degraded. Therefore, the data pickup performance for the disk drive is lowered when speeding up. A trial-and-error way is used to solve the problem by setting the disk at a maximum speed at first. Once the disk data cannot be normally accessed at such high speed, the angular speed of the disk is stepwise lowered until an acceptable value, i.e. an optimal speed, is reached. Then, the disk data is accessed at that optimal speed under the control of the servo control system. The time period the user has to wait for the trial-and-error procedure to enter the data pickup operation, however, is relatively long for today's requirement on speed.